Aortocaval compression is a well-recognized problem that can occur during pregnancy. When pregnant women assume a supine position, the enlarged uterus can compress the maternal aorta and/or inferior vena cava (IVC compression). FIG. 1 illustrates a pregnant woman 1 lying in a non-tilted supine orientation, wherein the uterus 2 carrying the fetus 3 bears on and compresses the maternal aorta and/or inferior vena cava, indicated collectively at 4. This vascular compression can potentially compromise maternal hemodynamics and cause uteroplacental hypoperfusion. This condition, which is often referred to as supine hypotensive syndrome or vena cava syndrome, is not only potentially dangerous to the expectant mother, but can also have devastating consequences to the developing fetus. Since uteroplacental blood flow is directly correlated to maternal perfusion pressure, sustained aortocaval compression can cause fetal hypoxia and acidosis.
To avoid aortocaval compression, women in late pregnancy are often encouraged to adopt a left-lateral tilt position when lying down. The left-lateral tilt position is generally preferred over the right-lateral tilt position, as it has been shown that left-lateral positioning is less likely to cause compression of the inferior vena cava (although lateral tilt in any direction is generally better than supine positioning). For example, FIG. 2A illustrates a pregnant woman 1 lying in a left-lateral tilt position, and FIG. 2B illustrates a cross-section showing the corresponding position of the uterus 2 and fetus 3, wherein the uterus and fetus are shifted laterally with respect to the maternal aorta 5, inferior vena cava 6, and spinal structure 7, thereby relieving aortocaval compression caused by the fetus.
The optimal tilt angle is ultimately dependent on maternal anatomy, gestational age, the position of the fetus within the uterus, and other factors, but it is commonly recommended that a lateral tilt angle of greater than 15 degrees should be used. At lateral tilt angles greater than 15 degrees, aortocaval compression is typically relieved, thereby increasing maternal cardiac output by restoring venous return to the heart.
Recently it has been shown that there is a strong association between maternal sleep position and rates of stillbirth and low birth weight. The first reports of this association were published in 2011 from a case-control study in Auckland. Investigators found that women who slept on their back were 2.5× more likely to have a stillbirth than woman who slept on their left side. These results were confirmed by a 2012 study in Ghana, where investigators found that the supine sleep position increased the probability of a stillbirth by 8× and the probability for low-birth weight by 5×. The findings were confirmed yet again by a 2015 study in Sydney, where investigators found a strong association between supine maternal sleep position and rates of late pregnancy stillbirth.
The association between maternal sleep position and stillbirth is consistent with the fact that aortocaval compression caused by the gravid uterus can impair uteroplacental perfusion and fetal oxygenation. Although significant aortocaval compression can cause maternal hypotension and tachycardia, aortocaval compression can also occur in the absence of any overt changes in maternal vital signs. However, even in the absence of overt vital sign abnormalities, there are measurable biometric signs that can indicate the occurrence of a physiologic derangement, such as caused by aortocaval compression.
It has been demonstrated that characteristic changes in cardiac autonomic nervous system activity occur during pregnancy, which become particularly obvious when pregnant women assume a supine or right lateral decubitus position. To compensate for the decrease in venous return and cardiac output that occurs as a result of aortocaval compression, vagal tone is suppressed and sympathetic tone is enhanced. These autonomic nervous system changes can cause characteristic changes in heart rate or heart rate variability (variation in the beat-to-beat intervals of the heart).